Thin film transistor liquid crystal displays (TFT-LCDs) have become dominant in the present flat plate display market due to their advantages of small volume, low power consumption, and no irradiation, and are widely applied in desktop computers, notebook computers, personal digital assistants (PDA), mobile telephones, TV sets and monitors. The manufacturing process of a TFT-LCD can be divided into approximately the following three stages:
I. Array Process
Individual TFT pixel array circuits are formed on a glass substrate by a plurality of mask processes, with each pixel array area corresponding to one liquid crystal display (panel) to form an array substrate (TFT substrate).
II. Cell Process
Liquid crystal is dropped on the TFT substrate and is covered by a color filter substrate such that an LCD panel is formed, and the LCD panel is cut to form individual liquid crystal displays.
III. Module Process
A backlight source, optical films and peripheral circuits are mounted for each liquid crystal display so as to form a complete TFT-LCD display module.
When the glass substrate is put into the array process, with a bottom gate TFT-LCD as an example, the first manufacturing step is typically to manufacture gate electrodes and gate lines. The first manufacturing step also forms alignment marks at edges and corners of the glass substrate at the same time, and the alignment marks are typically formed into the shape of cross and made of a metal film, hence are opaque. Alignment marks on the array substrate play a critical role in the above three manufacturing stages. In each manufacturing step, it is generally required to hold the glass substrate onto an apparatus correspondingly for the step (such as a sputtering apparatus, a PECVD (Plasma Enhanced Chemical Vapor Deposition) apparatus, an exposure apparatus, a track machine (namely other apparatuses other than the exposure apparatus in the photolithography process)) so that the glass substrate can undergo corresponding operations such as film formation, exposure, etching and so on. However, since it is possible that the glass substrate deviates from its standard position (namely the position the glass substrate should be located at in the ideal state of a holding member without error) when the holding members of various apparatuses hold the glass substrate, before the glass substrate is subjected to the corresponding operations, the glass substrate needs to be aligned to check whether the glass substrate is well aligned.
At present, an approach used to align the alignment marks on the glass substrate is as follows: saving or keeping in advance a standard picture of alignment marks on the glass substrate while the glass substrate is in the standard position; and in the manufacturing process, comparing the currently-taken picture of alignment marks on the glass substrate with the pre-saved standard picture if alignment is desired. If the comparison result is consistency, it indicates the glass substrate is aligned accurately and the glass substrate is subjected to the corresponding operations; in case of in-consistency, it indicates the glass substrate is not aligned accurately.
In summary, the current alignment approach used in the manufacturing process liquid crystal display devices suffers from long processing time and low processing efficiency.